Because under no stretch of the imagination is having sex with a horse within the constraints of socially acceptable behaviour - it's a non consenting animal.
Perhaps a nice strapping stallion should do the same to him...right.....taste of his own medicine ...march him off to the nearest stud farm
omg!!!! how sick can you get! and what made me the madest was this quote at the end... 
he should have his balls ripped off or better yet line him up behind some horses that like to buck and get them to kick him in the nuts till they fall off themselves! 
a senior policeman told me years ago that this is much commoner than we think. he'd headed a major surveillance campaign after a girl tending her pony was molested, and apparently the hidden coppers saw a surprising number of men regularly interfering with horses.
tbh, i'm not sanctioning it at all, God forbid, BUT for the horse how is it that different to a rectal examination which a vet does, where they put their whole arm in? that CANNOT be comfortable either. I doubt a mare can feel a man's *****! the thing is, we train our horses to put up with whatever we choose to do to them, we expect them to accept a vet sticking heaven-knows-what inside them, a horse isn't to know that a man isn't supposed to do that.
he's sick and needs help but maybe i'm alone in thinking "thank heavens he did it to a horse - which probably couldn't feel it, and has no emotional trauma, no shame, etc - and not to a child..."
I am NOT defending him though, the sicko needs help.
I have only just read this and whilst reading through all the replies I must admit I have had the odd giggle at some of the responses. However, I am quite shocked that people are actually likening this to, veterinary examinations, or breeding, AI etc. I a furious tbh that some of you are actually saying that because potentially there is no psycological damage or internal damage it may not be that bad! The simple facts are as follows:
Examinations, AI and Natural breeding are infact done for some 'normal' purpose, examinations normally have some medical basis and the animals welfare at heart, none of these are done for 'sexual gratification' by sick people. The man in question was ABUSING a horse, it does not matter whether it was a horse, sheep, child, adult or labrador... he was abusing it for his own sick kicks. He can be as sick as he likes in my book, if he doesn't want to ask for consent then he can go use an inanimate object but not a living breathing animal, this is ABUSE. If a child goes into the doctors and gets touched inappropriately by the doctor but has no physical or psychological trauma from the event because they do not know that the process was wrong, does that negate the abuse? NO IT DOESN'T!
It is true that in america this is more common, but it is still infact illeagal in most states, I believe there are three states where it is not, the general consensus of opinion is that the sick freaks will congregate there, sort of like ghettoing them. The only Jerry Springer show never to have been aired due to being deemed to contraversial was infact about a man and his horse (I will leave you to fill in the gaps on the subject matter!) It doesn't actually matter how common this is, it is still wrong
I'd be very surprised if that's true and in North American we're always told that abusing/harming (slashing etc.) horses is actually more common in the UK, rooted in your horsey culture. I'm not saying this it true but it's such human nature to say, "Ooh, those sick people do it but never US!" 
Apparently not true. There was both a semi-documentary made about the subject and a movie based on a specific case made about it, which probably made for a bit more open discussion but I know people with stories from Germany, New Zealand, South Africa etc so I don't think it's an American thing . . .
That's just being alarmist. Serial rapists/killers who "start" on animals do so with the intention of torturing them and intentionally causing them pain and suffering - that's how they get their kicks, not the actual act of sex - so not at all the case here.
As far as people having sex with farm animals . . . um, that's not exactly an uncommon idea, is it? How many jokes are there about it? (A pretty good indicator, since jokes rely on common knowledge.) And horses are a particular case, historically and socially.
None of which excuses him nor mitigates the need to sort him out in whatever way we currently deem socially acceptable. And no one should be able to harm an animal in any way for their own ends and get away with it. (Although that's a can of worms . . . ) But the idea that this is some one off and his next stop will be the local primary school doesn't have much basis in reality.
As far as it being "more common in America" - cite your stats, people.